In this study, we investigated the cost-effectiveness of four alternatives: Liquified Natural Gas (LNG), methanol, green hydrogen, and green ammonia, for the case of top 20 most frequently calling ships to Irish ports in 2019, through the Net Present Value (NPV) methodology, incor- porating the benefits incurred through saved external, carbon tax and conventional fuel costs. LNG had the highest NPV (€6,166 million), followed by methanol (€1,705 million) and green hydrogen (€319 million). Green ammonia utilisation (as a hydrogen carrier) looks inviable due to higher operational costs, resulting from its excessive consumption (i.e., losses) during the cracking and purifying processes and its lower net calorific value. Green hydrogen remains the best option to meet future decarbonisation targets, although, a further reduction in its current fuel price (by 60%) or a significant increment in the proposed carbon tax rate (by 275%) will be required to improve its cost-competitiveness over LNG and methanol.